Commercial grade sodium dithionite (Na.sub.2 S.sub.2 O.sub.4) (also called sodium hydrosulfite) generally has an assay of approximately 90 percent. Some of the impurities making up the other 10 percent have the effect of lowering the thermal stability of the product. For example, the temperature at which very pure sodium dithionite begins exothermic decomposition, as measured by Accelerating Rate Calorimetry, is approximately 150 degrees C. The typical temperature at which commercial grade sodium dithionite decomposes is around 80 degrees C. An increase in the concentration of certain impurities (such as water) can cause the temperature at which thermal decomposition begins to be even lower. It is important that the temperature at which decomposition begins be as high as possible, since conditions during handling, storage, or shipping may cause the temperature of the material to become sufficiently elevated to initiate rapid exothermic decomposition and fire. Sample temperature elevation may be caused by high ambient temperature such as that which occurs in warehouses or trucks during hot weather. Sample temperature elevation may also be caused by water contamination. Even a small increase in the temperature at which decomposition begins can improve the overall stability of a sample of sodium dithionite.
It is believed that a major cause of the lowered thermal stability of some sodium dithionite materials are proton donors (such as Bronsted acids) or substances which can increase the availability of protons in the sodium dithionite. In commercial grade sodium dithionite the most common source of protons is water. However, sodium metabisulfite is also a major impurity (usually around 6 percent). Sodium metabisulfite decomposes to produce sulfur dioxide (SO.sub.2) which reacts with water to form sulfurous acid, thus increasing the availability of protons. The partial pressure of SO.sub.2 formed from sodium metabisulfite increases with an increase in temperature, causing an increase in acidity and in turn a decrease in the thermal stability of sodium dithionite. Also, SO.sub.2 is a decomposition product of sodium dithionite, which means that in the presence of even small amounts of water the decomposition is autocatalytic. Historically, many different additives have been tried in an effort to improve the stability of sodium dithionite. All of these additives have been solids, with sodium carbonate (Na.sub.2 CO.sub.3) being the most widely used.
U.S. Pat. No. 3,054,658 to Franklin et al describes the use of sodium or potassium salts of carboxylic acids to improve the stability of Na.sub.2 S.sub.2 O.sub.4.
Japanese Patent No. 43-24774 to Noguchi describes a method for improving the stability of Na.sub.2 S.sub.2 O.sub.4 which includes the use of recrystallization, filtration and drying steps and the use of surface-active agents such as sodium dodecyl benzene sulfate sodium stearate and sodium sulfite.
British Patent No. 1,040,687 describes the use of suberic acid, azelaic acid, sebacic acid (including salts thereof and mixtures thereof) as improving the stability of sodium dithionite toward air oxidation at room temperature.
U.S. Pat. No. 3,287,276 to Poschmann et al describes the use of water soluble macromolecular substances as additives to inhibit the decomposition of sodium dithionite. Examples of such substances include poly(meth)acrylic acid, poly(meth)acrylamide, polyvinyl alcohol, polyethylenimine, polyvinylpyrrolidone and water soluble polyacetals.
Japanese Patent No. 1971-16,659 to Fujiwara et al describes the use of alkylene oxides such as ethylene oxide, propylene oxide or beta-butylene oxide to improve the stability of sodium dithionite against decomposition when exposed to air and moisture.
Japanese Patent No. 1971-38,408 to Toda et al claims that sodium dithionite can be stabilized by washing it with a methanol solution of sulfur dioxide or sulfurous acid.
British Patent No. 1,259,121 describes the use of amphoteric or nonionic surfactants such as polyoxyethylene alkyl esters, polyoxyethylene alkaryl and alicyclic aryl ethers to stabilize sodium dithionite against loss of assay upon storage.
U.S. Pat. No. 3,666,400 and British Patent No. 1,262,560 to Lofton et al. describe the use of amines or quaternary aliphatic ammonium salts to improve the storage stability of sodium dithionite. The patent claims that the amine functional group serves to scavenge protons on the surface of the dithionite while the alkyl chain makes the sodium dithionite hydrophobic.
British Patent No. 1,287,699 to Mitsui Toatsu Chemicals Inc. claims that coating the sodium dithionite with oxypropylated cellulose or oxypropylated starch at the stated levels improves the storage stability of the sodium dithionite.
U.S. Pat. No. 3,794,738 to Ellis et al. discusses the addition of alkali metal and ammonium salts of diglycolic acid to stabilize sodium dithionite against self-ignition when the sample is contaminated with water.
British Patent No. 1,374,029 to BASF Aktiengesellschaft utilizes a combination of anhydrous sodium carbonate and penta-sodium diethylene triaminopentaacetate to stabilize sodium dithionite against self-ignition when contaminated with water.
U.S. Pat. No. 3,856,696 and British Patent No. 1,415,837 to Stanbank et al. describe the addition of an unsaturated carboxylic acid or anhydride thereof to stabilize alkali and alkaline metal dithionites against self-ignition. Preferred acids are aryloxy alkanoic acids.
U.S. Pat. No. 3,923,960 and British Patent No. 1,448,208 to Leigh teaches that an anhydrous dithionite composition may be rendered resistant to ignition (after contamination with water) by adding a carboxylic acid salt of a primary, secondary or tertiary amine having at least one hydrocarbon group of at least five carbon atoms.
U.S. Pat. No. 4,108,960 and British Patent No. 1,469,234 to Leigh describe the use of aromatic carbonyl compounds such as vanillin, ethylvanillin or benzoin to stabilize metal dithionites against self-ignition.
Additives in current use are limited in their ability to remove water and other impurities such as sulfur dioxide which can cause an increase in the availability of protons. Additionally, throughout the previous attempts to find stabilizers for sodium dithionite it has been recognized that sodium carbonate alone is not sufficient as a single additive. The number and diversity of the compounds described above indicates that there remains a need for more effective ways of stabilizing sodium dithionite. Thus, it is an object of the present invention to provide stabilized sodium dithionite materials so that it is less likely to undergo rapid exothermic decomposition when exposed to air and moisture. It is another object of this invention to stabilize commercial grade sodium dithionite so that it is less likely to burn if the temperature becomes elevated during storage. It is a further object of this invention to provide such stabilized materials with the use of relatively inexpensive additives. These and further objects of the invention will become apparent from the following description of the invention.